Can anybody tell how to find out when and how and who have inserted data in a given db2 table ? We have bmc catalog manager in our shop, but it shows there is no load job that ran and loaded data but table still has data.

It is possible to get it from SMF, but you still would need MXG or MICS or some other similar software that processes SMF.
DB2ACCT - MXG dataset that has the number of SQL creates, deletes, inserts, updates information.